Crystal growth sampler



A1181 1967 R. E. LORENZINI CRYSTAL GROWTH SAMPLER 2 Sheets-Sheet 1 FiledMarch 15, 1965 ATTORNEYS ROBE? T f. LORENZ/All F I E 3 INVENTOR.

FIE--1- United States Patent 3,337,304 CRYSTAL GROWTH SAMPLER Robert E.Lorenzini, Mountain View, Calif., assignor to Elmat Corporation, acorporation of California Filed Mar. 15, 1965, Ser. No. 439,813 2Claims. (Cl. 23273) This invention relates to an improvement in acrystal growing furnace and to a method of operating such a furnace.More particularly, the invention relates to a crystal growing furnacehaving a unique double window structure therein and the method ofutilizing such a furnace wherein one or more samples are withdrawn andanalyzed before a crystal is grown to insure that the ultimate crystalwill be of the desired chemical composition and will contain a minimumnumber of dislocations.

It is an object of the present invention to provide a crystal growingfurnace having a double window therein so that the window can be keptclear at all times.

A further object of this invention is to provide a double acting windowwherein the window can be used to seal both an opening of a crystalgrowing furnace and to seal off a portion of the crystal growingchamber, permitting analytical samples to be taken during the course ofa melt.

Another object of this invention is to provide a melt sampling methodwhereby one is insured that the ultimate crystal which is grown is of adesired, definite chemical composition.

Still another object of this invention is to provide an interlock systemwhereby the crucible may be recharged with material without having tocool the furnace and disassemble it, thereby increasing the amount ofcrystal produced per unit time.

Other objects and features of the advantage of the invention will beapparent from the specification which follows.

In the drawings forming part of this application:

FIGURE 1 is a perspective View of a crystal growing furnace embodyingthe present invention and showing. a crystal being grown. I

FIGURE 2 is a section on the line 2-2 of FIGURE 1.

FIGURE 3 is a section generally on the line 33 of FIGURE 2 but showingthe position of the parts when a sample is being taken.

FIGURE 4 is a perspective view of a sample and resistance probe showingan analytical method which may be used.

FIGURE 5 is a view similar to FIGURE 3 but illustrating the method ofrecharging the crucible.

The basic crystal growing furnace of the present invention is describedand claimed in a copending patent application and the basic structure,therefore, will not be here described in detail. Suffice it to say thatthe present invention is applicable to a crystal growing apparatusembodying a double-walled vessel 56 having a relatively large lowerportion 57 and an elongated neck portion 59. Within the chamber a quartzcrucible 75 is contained in a graphite holder 77, the latter beingmounted for rotation on a tubular member 79. The crucible is surroundedby a resistance heating element 103, which is mounted on brackets 105and 107 attached to rods 109 and 111, respectively, it being understoodthat the rods and brackets serve the dual purpose of mechanicallysupporting the heater and also supplying electricity thereto. Further,means are provided for moving the rods so that the heating element canbe lowered as the surface of the melt becomes lower due to the growth ofa crystal. The double-walled vessel is supplied with a cooling fluidsuch as Water which flows between the walls of the vessel.

The water inlet and outlet have been omitted from the drawings for thesake of simplicity. The interior of the vessel is flushed with an inertgas such as argon which can be introduced through line 71 to either thetop portion 59 or the bottom portion 57 or both by appropriatemanipulating of valves 71A and 71B. Valve 71A is normally open, whilevalve 71B is normally closed. Gas can be exhausted through line 73,which is open at all times or may be exhausted through line 190 byopening valve 190A, as is hereinafter explained. The base of the vessel56 is fastened to a furnace base member 65 which rests on a supportmember 17, the vessel being held in substantially air-tight relationshipby means of the clamps 69. It will be understood that the rods 1-09 and111 as well as tube 79 pass through base 65 in sealing relationship. Acrystal growing rod 49, to which is attached a seed crystal 50, extendsdownwardly through the vessel 59 into the vessel 57.

The parts thus far described are parts which are common to the parts setforth in my copending patent application and only sufficient detail hasbeen given to enable one to understand the present invention. Thus,means are provided for rotating and lowering, and ultimately slowlyraising, the crystal growth rod 49; for rotating the crucible and forlowering the heating element as crystal growth proceeds.

The upper chamber 59 has a top 188 held by clamps 188A and at the centerhas a Teflon bushing 189 through which rod 49 can pass. The lowersection 57 and the upper section 59 of chamber 56 are interconnected bya flat, horizontal portion having a composite window, generallydesignated 151, therein. The window 151 has a lower sealing member 155and an upper sealing member 157. At least the upper, and preferably bothwindows have a center, transparent portion, preferably of quartz. Thelower sealing member 155 has a groove 159 which corresponds in diameterboth to a lip 161 which extends downwardly from the inner wall of theupper portion of the vessel 59 and also a similar lip 163 which extendsdownwardly from the window aperture. In addition, groove 159 has asealing gasket 165 therein. The window 155 is pivoted on the pin 167,the outer end of the pin terminating in a handle 169'. Thus the window155 can be turned to a first position where it acts as a seal for thewindow 151, to a second position where it acts as a seal for the topportion of the vessel 59 (see FIGURE 3) or to a third or intermediateposition (as in dash lines in FIG- .URE 2) between the first andsecondpositions. Window element 157 has a downturned rim 171 so that it iscapable of sealing the window aperture from the top.

The double window arrangement serves three useful purposes inconjunction with the making of crystals. In the first place, thecrystalline materials, such as silicon and germanium, vaporize and thevapor condenses on .any relatively cool surface such as a window andsoon forms a deposit which makes the window opaque. The double windowarrangement permits one to always have a clear window without permittingthe contamination which might ensue if one merely lifted out a windowand cleaned it. Thus, if the window 157 becomes opaque due to thedeposit of material on the inner surface thereof, one merely swingswindow 155 into sealing position on lip 163 of the window aperture.Window 157 can be removed, cleaned and replaced without danger ofcontaminating the interior of the crystallization vessel. After thewindow 157 is replaced, the window 155 is swung out of the way and theclean window 157 is now used for observation purposes.

Another important function of the window is illustrated in FIGURES 3 and4. In making a crystal of a material such as hyperpure silicon to whichhas been added a small amount of a doping material, the composition ofthe crystal will bear a known relationship to the composition of themelt. Since it is most advantageous to know the precise composition ofthe melt and therefore of the resultant crystal, I herein descn'be amethod whereby this invention can be used to monitor the composition ofthe melt without contaminating the melt with air or other gases. Thus,before crystal formation is started, the Window 155 can be moved to thesealing position on lip 161 shown in FIGURE 3. At the same time, theflow of inert gas from line 71 is diverted by closing valve 71A andopening valve 713. The seed shaft is then removed and replaced by aquartz tube 177 of like dimension so as to form a good seal throughTeflon bushing 189. Valve 190 on the upper exhaust port (normallyclosed) is then opened and window 155 is moved aside. Quartz tube 177 isthen lowered into the molten material, with bulb 179 being compressedand a sample (181) of material is drawn into the tube. Tube 177 is thenpulled up into chamber 59 and window 155 is again sealed on lip 161.After being allowed to cool for approximately 5 minutes, tube 177 iscompletely withdrawn from top 188. Tube 177, together with thecrystalline material 181, which solidifies therein, is ground andpolished toa flat surface as at 183 and 187, A resistance probe 185 isthen applied to the surface 187 of the crystalline material and aresistance reading taken. Since pure silicon has substantially aninfinite resistance and since the chemical nature of the dopingmaterials which lend conductivity to the silicon are known, theresistance of the sample thus taken will give an accurate indication ofthe chemical composition of the upper surface of the melt. If thecomposition is correct, seed rod 49 is inserted through bushing 189,valve 71A is opened, valve 71B is closed and chamber 59 is allowed topurge with inert gas which has entered through valve 71A into chamber 59and subsequently exhausts through line 190. After the purge cycle,window 155 is moved aside and valve 190A is closed, thereby allowing thenormal gas flow through valve71A, downwards through chambers 59 and 57and out exhaust 73.

Seed rod 49 with seed 50 attached, is dipped into the surface of themelt and slowly withdrawn to form a large crystal 153 as shown in FIGURE1.

If the composition after sampling was not correct, another quartz tubesimilar to 177 is inserted into the melt as explained in the samplingprocess. However, instead of withdrawing a sample of melt, pellets ofdoping chemicals are dropped into the tube in order to change thechemical composition of the melt to that desired. The sampling anddoping procedure can then be repeated until the desired composition isattained.

A third useful purpose of the double window arrangement is to allow themelt to be replenished with material without turning the furnace oflf.This is illustrated in FIG- URE 5. Historically, crystal growing hasbeen a batch process whereby one crystal was grown from a cruciblecharge, the furnace turned off and allowed to cool, the chamberdisassembled and cleaned and a new charge placed in the crucible. Thisprocess resulted in a considerable amount of unproductive time spent onthe furnace and also resulted in the loss of a crucible after each runsince the small amount of silicon left in the crucible expands duringfreezing and breaks every crucible.

By recharging the crucible while the furnace is hot, a considerablesavings in both of the above areas takes place.

To recharge the furnace valve 71B and 190A are opened, 71A is closed,and window is brought into position to seal on lip 161. Top 188 isremoved and stainless steel tube 191 is inserted in bushing 189. Tube191 includes a center rod 192 which terminates at its upper end in aknob 193 and at its lower end in three or more incurved fingers 194which are biased in grasping relationship by spring 195. The fingers areexpanded in order to grasp a large piece of silicon 196. The entireassembly is clamped back onto chamber 59, valve 71A is opened, 71B isclosed and chamber 59 allowed to purge. Valve A is then closed whilewindow 155 is moved aside. Tube 191 with silicon attached is moved downinto crucible 75, knob 193 is depressed and the silicon 196 dropped intothe mother liquid. Tube 191 is then withdrawn and replaced 'by seed rod49 as per same procedure as the sampling technique.

Although a specific embodiment of the invention has been illustrated, itwill be understood by those skilled in the art that this is for purposesof illustration only and that departures may be made in the exactstructure and method without departing from the spirit of the invention.For instance, a specific rotating handle and pin arrangement has beenshown for moving the interior window to its various positions; othermeans of moving the window might be employed.

I claim:

1. In a crystal growing furnace wherein a crystal is grown in anenclosed crystallization vessel from a melt, said crystallization vesselhaving a crucible with heating means in surrounding relationshiptherewith and having a reduced diameter portion extending upwardly fromthe vessel, means for inserting a crystal growing rod through thereduced diameter portion overlying the crucible and for withdrawing saidrod and growing a crystal thereon, said reduced diameter portion havinga horizontally disposed opening with a sealing lip extending around theperiphery of said opening, a composite window in the crystallizationvessel, said window comprising an aperture in the vessel, said aperturehaving a sealing lip of substantially the same diameter and locatedhorizontally adjacent the said opening, an interior window movablewithin said vessel, the interior window being movable by means of anexternal rotary handle to seal selectively the aperture or the opening.

2. The structure of claim 1 wherein said aperture has an exterior windowwhich can be removed or inserted in said aperture whereby the exteriorwindow can be removed and cleaned when the interior window is in sealingrelationship with said aperture.

References Cited UNITED STATES PATENTS 2,908,254 10/1959 Mangold 88-13,145,705 8/1964 Bruce 88-1 3,249,005 5/1966 Foguel 88l NORMAN YUDKOPF,Primary Examiner.

WILBUR L. BASCOMB, Examiner.

G. HINES, Assistant Examiner,

1. IN A CRYSTAL GROWING FURNACE WHEREIN A CRYSTAL IS GROWN IN ANENCLOSED CRYSTALLIZATION VESSEL FROM A MELT, SAID CRYSTALLIZATION VESSELHAVING A CRUCIBLE WITH HEATING MEANS IN SURROUNDING RELATIONSHIPTHEREWITH AND HAVING A REDUCED DIAMETER PORTION EXTENDING UPWARDLY FROMTHE VESSEL, MEANS FOR INSERTING A CRYSTAL GROWING ROD THROUGH THEREDUCED DIAMETER PORTION OVERLYING THE CRUCIBLE AND FOR WITHDRAWING SAIDROD AND GROWING A CRYSTAL THEREON, SAID REDUCED DIAMETER PORTION HAVINGA HORIZONTALLY DISPOSED OPENING WITH A SEALING LIP EXTENDING AROUND THEPERIPHERY OF SAID OPENING, A COMPOSITE WINDOW IN THE CRYSTALLIZATIONVESSEL, SAID WINDOW COMPRISING AN APERTURE IN THE VESSEL, SAID APERTUREHAVING A SEALING LIP OF SUBSTANTIALLY THE SAME DIAMETER AND LOCATEDHORIZONTALLY ADJACENT THE SAID OPENING, AN INTERIOR WINDOW MOVABLEWITHIN SAID VESSEL, THE INTERIOR WINDOW BEING MOVABLE